1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system, such as a copying machine, printer, facsimile device, or a composite device of these, and to a process cartridge installed in same, and more particularly, to an image forming apparatus and process cartridge which use a two-component developer comprising a toner and carrier.
2. Description of the Background Art
Conventionally, in an electrophotographic image forming apparatus, or the like, a magnetic brush developing system using a two-component developer comprising a magnetic carrier and a toner is adopted in order to develop an electrostatic latent image formed on a latent image carrier. Normally, a developing device based on this system comprises an internally provided magnetic roller made of a magnetic body having a plurality of magnetic poles, a frame which accommodates the magnetic roller, and a developing sleeve which is a cylindrical developer carrier that is supported in a rotatable fashion. Magnetic carrier to which toner is attached is held by the magnetic force of the magnetic roller on the surface of the developing sleeve, and it is conveyed to a developing region where developing is carried out. Furthermore, in order to improve the fluid characteristics of the toner, fine non-organic particles of small particle size, such as silica, are added to the toner as an external additive.
Due to recent demands for improved image quality in image forming apparatuses, there has been a tendency to reduce the particle size of the toner and the magnetic carrier, and reduce the interval between the photosensitive body forming the image carrier and the developing roller forming the developer carrier (hereafter, this interval is called the “developing gap”). However, as the particle sizes of the toner and the carrier become smaller, so the surface area of the particles becomes greater with respect to the mass of the toner and carrier. Consequently, contact becomes more liable to occur between respective toner particles, or between toner particles and carrier particles, and the resulting friction is liable to degrade the fluidity. Furthermore, since the indentations in the surface of the toner also become smaller, then a phenomenon occurs whereby the toner additive, such as silica, which serves to improve the fluidity of the toner, becomes embedded in the surface of the toner, and hence deterioration of the fluidity over time becomes more liable to occur. This deterioration of the fluidity impedes the dispersion of the toner within the developer, and hence insufficiently charged toner or inversely charged toner arises, which ultimately leads to soiling of the bare surface regions of the image. Furthermore, in a state of degraded fluidity, aggregation of toner also occurs, and soiling due to large particles on the bare surface regions also present a serious image defect.
In principle, the toner additive having a small particle size enters in between respective toner particles and between toner particles and magnetic carrier particles, and prevents the microparticles of toner and magnetic carrier from adhering tightly to each other, thereby preventing increase in intermolecular forces, reducing the adhesive force, and hence serving to increase fluidity. By improving fluidity, the occurrence of aggregated toner particles, or undercharged toner or inversely charged toner due to impeded dispersion of the toner, is reduced, and hence it is possible to reduce the occurrence of soiling caused by same, and the like. However, if the phenomenon of embedding of the additive occurs, then this promotes adhesion between toner particles and between toner and magnetic carrier particles, and hence the aforementioned fluidity declines. If the fluidity declines, then aggregation of toner and soiling of the bare image surface is liable to occur.
Embedding of the additive is a phenomenon which occurs during the churning action inside the developing device. Therefore, the greater the number of images formed, the longer the churning time and the greater the amount of additive that becomes embedded.
It is known that an effective method of reducing the amount of additive which becomes embedded when using toner of small particle size of this kind involves adding both silica forming an additive which serves to improve fluidity (hereinafter, called “small-particle silica”) and large-particle silica having a larger particle size than the small-particle silica (hereinafter, called “large-particle silica”), as disclosed in Japanese Patent Laid-open No. 2000-81723, for example.
In general, silica added in order to improve fluidity has high hardness compared to the toner particles, and its particle size is sufficiently small compared to the particle size of the toner particles and carrier particles and the surface area of the contact surfaces between the silica and the toner particles is also small. If the surface area of the contact surface is small, then when force is applied, the pressure will not be dispersed readily and hence the silica is liable to become embedded into the toner particles, which are softer than the silica. Consequently, if only small-particle silica is added, then when a toner particle collides with a carrier particle and the force of this impact is applied to the silica, then the small-particle silica situated between the toner particle and the carrier particle will readily become embedded in the toner particle. Embedding of this kind occurs not only between a toner particle and a carrier particle, but also between respective toner particles.
On the other hand, if both large-particle silica and small-particle silica are used conjointly, then since the large-particle silica has a larger particle size than the small-particle silica, the surface area of the contact surface with the toner is increased. If the surface area of the contact surface is large, then even if a toner particle and carrier particle collide and a force of similar magnitude to that received by the small-particle silica described above is applied to the silica, the resulting pressure is dispersed and hence the silica is not liable to become embedded. As a result, the large-particle silica serves as a spacer. Since the large-particle silica serves as a spacer, it is possible to suppress the silica embedding action caused by small-particle silica located between a toner particle and a carrier particle, or between two toner particles.
However, if the added amount of large-particle silica is too large with respect to the toner, then not all of the large-particle silica will adhere to the surface of the toner particles, and the surplus silica will cause filming on the surface of the photosensitive body. On the other hand, if the added amount of large-particle silica is too small, then it will not serve adequately as a spacer and it will not be possible to prevent embedding of the small-particle silica, thus leading to deterioration of toner fluidity. Moreover, if the added amount of small-particle silica is too large, then the surplus silica will cause filming on the photosensitive body and if the added amount of small-particle silica is too small, then this will lead to deterioration of toner fluidity.
Due to these reasons, determining the amount of additive added with respect to the toner is important in forming desirable images. In Japanese Patent Laid-open No. 2000-81723, added amounts which enable desirable images to be formed are stipulated in respect of both the large-particle silica and the small-particle silica. However, the image forming apparatus described in Japanese Patent Laid-open No. 2000-81723 uses a non-magnetic one-component developer only, and does not investigate a two-component developer which uses a toner and a carrier.
Furthermore, if the particle size of the toner is reduced, then the surface area increases with respect to the weight of the toner, and therefore, if the charge density on the surface is uniform, the amount of charge per unit mass (Q/M) increases. If the charge rises excessively, then the electrostatic charge of the magnetic carrier is spent by the toner particles in the high-charge region of the charge distribution, and uncharged toner, which has been supplied more recently, does not receive a sufficient charge, leading in turn to toner scattering, soiling, and other problems. This issue is particularly notable in low-humidity environments where frictional charging occurs more readily.
On the other hand, Japanese Patent Laid-open No. 2004-212560 describes an image forming apparatus, such as a color copying device, color printer, or the like, using a two-component developer comprising a toner and a carrier, as described above, in which a developing step is performed by applying only a DC developing bias to a developing sleeve which holds the two-component developer.
A developing method using a two-component developer is considered to produce better and more stable quality in the output image than a developing method using a one-component developer, because the charging of the toner is stabilized. Furthermore, a developing method which applies only a DC developing bias to a developing sleeve allows the composition and control procedure of the power supply unit to be simplified, and hence reduces device costs, in comparison with a developing method which applies both a DC and an AC developing bias, or a developing method which applies only an AC developing bias. What is more, it is less liable to give rise to blurred images as a result of carrier particles having low resistance.
Japanese Patent Laid-open No. 2004-212560 described above discloses technology for an image forming apparatus which adopts a developing method that uses a two-component developer and applies only a DC developing bias, and which uses a carrier of small size as the carrier in the two-component developer, in order to achieve high image quality. Consequently, the occurrence of adhesion of carrier particles is reduced, and the occurrence of blurred images or loss of peripheral areas of text is also reduced. More specifically, by optimizing the static resistance and saturation magnetization of the carrier when using a small-particle carrier having a weight-average size of 20 to 60 μm, the aforementioned problems are diminished.
Furthermore, the technology in Japanese Patent Laid-open No. 2004-212560 described above is able to reduce the occurrence of blurred images and the loss of peripheral regions of text, as well as reducing adherence of the carrier to edge portions of the toner image formed on the image carrier, such as the photosensitive drum, but there are cases where it is not able to suppress adherence of carrier to solid portions of the toner image, adequately. In particular, if the photosensitive drum and the developing device (developing sleeve, and the like) are reduced in size as the image forming apparatus is compactified, then adherence of carrier to the solid portions becomes much more liable to appear.
A more detailed description of the adherence of carrier particles to the edge portions and the solid portions is given below.
In other words, as described above, adherence of the carrier to the photosensitive drum includes adherence of the carrier to the edge portions of the toner image on the photosensitive drum (hereinafter, called “adherence of carrier to edge portions”) and adherence of the carrier to the solid portions of the toner image on the photosensitive drum (hereinafter, called “adherence of carrier to solid portions”).
Adherence of carrier to the edge portions is a phenomenon in which carrier adheres to the edge portions of the toner image on the photosensitive drum (in other words, the boundary between the image section and the non-image section) due to the counter-charge of the carrier. In the image section (toner image) on the photosensitive drum, an electric field is formed in a direction which moves the toner from the developing sleeve and onto the photosensitive drum. On the other hand, in the non-image section (bare surface section) on the photosensitive drum, an electric field is formed in the opposite direction to the direction of movement of the toner from the developing sleeve onto the photosensitive drum. Therefore, at the edge portions, an electric field (called an “edge electric field”) is formed, in which the electric field acting in the aforementioned opposite direction is accentuated. In a region where an “edge electric field” of this kind is acting, the carrier moves onto the photosensitive drum and adheres to the drum, due to the counter-charge which remains on the surface of the carrier after movement of the toner. This adherence of carrier to the edge portions is a phenomenon which becomes more notable, the greater the resistance of the carrier.
On the other hand, adherence of carrier to the solid portions is a phenomenon in which carrier adheres to the solid portions of the toner image on the photosensitive drum (the solid image portions), due to electrical charge induced electrostatically in the carrier. The adherence of carrier to the solid portions is particularly liable to occur in cases where the developing potential of the solid portion (in other words, the electric field potential formed in the image section) is high, or where the surface potential (in other words, the electric field potential in the opposite direction, which is formed in the non-image section) is high, or the resistance of the carrier is low.
In this respect, it has been considered that adherence of carrier to the solid portions can be reduced by adjusting the developing potential and the surface potential. However, any adjustment of the developing potential and the surface potential has a direct affect on image quality characteristics, such as image density, surface soiling, and the like, and therefore, such adjustment is subject to limitations. Furthermore, it has also been considered that adherence of carrier to the solid portions can be reduced by setting the carrier resistance to a high value. However, setting a high carrier resistance runs counter to measures for reducing adherence of carrier to the edge portions described above. In other words, if the carrier resistance is set to a high value, then although this reduces adherence of carrier to the solid portions, the adherence of carrier to the edge portions becomes more pronounced.
On the other hand, as described above, recently there have been strong demands for reduced size and higher image quality in image forming apparatuses, and in order to reduce the size of an image forming apparatus, it is necessary to reduce the size of the photosensitive drum, developing sleeve, and the like. However, if the external diameter of the photosensitive drum and developing sleeve is reduced, then on the downstream side in the direction of rotation from the position at which the drum and sleeve oppose each other (in other words, the developing region), there will be a reduction in the magnetic constriction force acting on the carrier at the tip of the magnetic brush created by the two-component developer held on a developing sleeve. Therefore, in addition to adherence of carrier to the edge portions, adherence of carrier to the solid portions also becomes more liable to occur.
In response to this, it has been considered that reduction of the magnetic constriction force acting on the carrier can be offset by setting the saturation magnetization of the carrier to a high value. However, since there is a certain degree of correlation between the saturation magnetization of the carrier and its resistance (namely, the fact that the resistance tends to decrease as the saturation magnetization becomes higher), then there are also limitations on the adjustment of the saturation magnetization.
Furthermore, in order to achieve high image quality, as described above, it is necessary to reduce the particle size of the toner while also reducing the particle size of the carrier. However, if the size of the carrier particles is reduced, then the magnetic force acting on each carrier particle becomes smaller, and therefore, adherence of carrier to the solid portions becomes more liable to occur in addition to adherence of carrier to the edge portions. Japanese Patent Laid-open No. 2004-212560 described above, and other references, specify conditions for the small-diameter carrier (in other words, static resistance, saturation magnetization, and the like), in order to reduce the occurrence of secondary effects, such as image blurring and loss of the peripheral region of text, and so on. However, adequate settings are not provided in respect of small-diameter carrier conditions for reducing the occurrence of adherence of carrier to the solid portions.
If adherence of carrier to the solid portions and adherence of carrier to the edge portions occurs, the members such as the cleaning blade and the intermediate transfer belt, which make contact with the photosensitive drum, become soiled by the adhering carrier particles, and these adhering carrier particles are transferred onto the transfer receiving medium, leading to blanking out of the image.